This invention relates to chemically synthesized non-nucleotide-containing enzymatic nucleic acid.
The following is a brief history of the discovery and activity of enzymatic RNA molecules or ribozymes. This history is not meant to be complete but is provided only for understanding of the invention that follows. This summary is not an admission that all of the work described below is prior art to the claimed invention.
Prior to the 1970s it was thought that all genes were direct linear representations of the proteins that they encoded. This simplistic view implied that all genes were like ticker tape messages, with each triplet of DNA xe2x80x9clettersxe2x80x9d representing one protein xe2x80x9cwordxe2x80x9d in the translation. Protein synthesis occurred by first transcribing a gene from DNA into RNA (letter for letter) and then translating the RNA into protein (three letters at a time). In the mid 1970s it was discovered that some genes were not exact, linear representations of the proteins that they encode. These genes were found to contain interruptions in the coding sequence which were removed from, or xe2x80x9cspliced, outxe2x80x9d of, the RNA before it became translated into protein. These interruptions in the coding sequence were given the name of intervening sequences (or introns) and the process of removing them from the RNA was termed splicing. At least three different mechanisms have been discovered for removing introns from RNA. Two of these splicing mechanisms involve the binding of multiple protein factors which then act to correctly cut and join the RNA. A third mechanism involves cutting and joining of the RNA by the intron itself, in what was the first discovery of catalytic RNA molecules.
Cech and colleagues were trying to understand how RNA splicing was accomplished in a single-celled pond organism called Tetrahymena thermophila. Cech proved that the intervening sequence RNA was acting as its own splicing factor to snip itself out of the surrounding RNA. Continuing studies in the early 1980""s served to elucidate the complicated structure of the Tetrahymena intron and to decipher the mechanism by which self-splicing occurs. Many research groups helped to demonstrate that the specific folding of the Tetrahymena intron is critical for bringing together the parts of the RNA that will be cut and spliced. Even after splicing is complete, the released intron maintains its catalytic structure. As a consequence, the released intron is capable of carrying out additional cleavage and splicing reactions on itself (to form intron circles). By 1986, Cech was able to show that a shortened form of the Tetrahymena intron could carry out a variety of cutting and joining reactions on other pieces of RNA. The demonstration proved that the Tetrahymena intron can act as a true enzyme: (i) each intron molecule was able to cut many substrate molecules while the intron molecule remained unchanged, and (ii) reactions were specific for RNA molecules that contained a unique sequence (CUCU) which allowed the intron to recognize and bind the RNA. Zaug and Cech coined the term xe2x80x9cribozymexe2x80x9d to describe any ribonucleic acid molecule that has enzyme-like properties.
Also in 1986, Cech showed that the RNA substrate sequence recognized by the Tetrahymena ribozyme could be changed by altering a sequence within the ribozyme itself. This property has led to the development of a number of site-specific dbozymes that have been individually designed to cleave at other RNA sequences.
The Tetrahymena intron is the most well-studied of what is now recognized as a large class of introns, Group I introns. The overall folded structure, including several sequence elements, is conserved among the Group I introns, as is the general mechanism of splicing. Like the Tetrahymena intron, some members of this class are catalytic, i.e., the intron itself is capable of the self-splicing reaction. Other Group I introns require additional (protein) factors, presumably to help the intron fold into and/or maintain its active structure.
Ribonuclease P (RNaseP) is an enzyme comprised of both RNA and protein components which are responsible for converting precursor tRNA molecules into their final form by trimming extra RNA off one of their ends. RNaseP activity has been found in all organisms tested. Sidney Altman and his colleagues showed that the RNA component of RNaseP is essential for its processing activity; however, they also showed that the protein component also was required for processing under their experimental conditions. After Cech""s discovery of self-splicing by the Tetrahymena intron, the requirement for both protein and RNA components in RNaseP was reexamined. In 1983, Altman and Pace showed that the RNA was the enzymatic component of the RNaseP complex. This demonstrated that an RNA molecule was capable of acting as a true enzyme, processing numerous tRNA molecules without itself undergoing any change.
The folded structure of RNaseP RNA has been determined, and while the sequence is not strictly conserved between RNAs from different organisms, this higher order structure is. It is thought that the protein component of the RNaseP complex may serve to stabilize the folded RNA in vivo.
Symons and colleagues identified two examples of a self-cleaving RNA that differed from other forms of catalytic RNA already reported. Symons was studying the propagation of the avocado sunblotch viroid (ASV), an RNA virus that infects avocado plants. Symons demonstrated that as little as 55 nucleotides of the ASV RNA was capable of folding in such a way as to cut itself into two pieces. It is thought that in vivo self-cleavage of these RNAs is responsible for cutting the RNA into single genome-length pieces during viral propagation. Symons discovered that variations on the minimal catalytic sequence from ASV could be found in a number of other plant pathogenic RNAs as well. Comparison of these sequences"" revealed a common structural design consisting of three stems and loops connected by a central loop containing many conserved (invariant from one RNA to the next) nucleotides. The predicted secondary structure for this catalytic RNA reminded the researchers of the head of a hammer; thus it was named as such.
Uhlenbeck was successful in separating the catalytic region of the ribozyme from that of the substrate. Thus, it became possible to assemble a hammerhead ribozyme from 2 (or 3) small synthetic RNAs. A 19-nucleotide catalytic region and a 24-nucleotide substrate were sufficient to support specific cleavage. The catalytic domain of numerous hammerhead ribozymes have now been studied by both the Uhlenbeck""s and Symons"" groups with regard to defining the nucleotides required for specific assembly and catalytic activity, and determining the rates of cleavage under various conditions.
Haseloff and Gedach showed it was possible to divide the domains of the hammerhead ribozyme in a different manner. By doing so, they placed most of the required sequences in the strand that did not get cut (the ribozyme) and only a required UH where H=C, A, or U in the strand that did get cut (the substrate). This resulted in a catalytic ribozyme that could be designed to cleave any UH RNA sequence embedded within a longer xe2x80x9csubstrate recognitionxe2x80x9d sequence. The specific cleavage of a long mRNA, in a predictable manner using several such hammerhead ribozymes, was reported in 1988.
One plant pathogen RNA (from the negative strand of the tobacco ringspot virus) undergoes self-cleavage but cannot be folded into the consensus hammerhead structure described above. Bruening and colleagues have independently identified a 50-nucleotide catalytic domain for this RNA. In 1990, Hampei and Tritz succeeded in dividing the catalytic domain into two parts that could act as substrate and ribozyme in a multiple-turnover, cutting reaction. As with the hammerhead ribozyme, the catalytic portion contains most of the sequences required for catalytic activity, while only a short sequence (GUC in this case) is required in the target. Hampel and Tritz described the folded structure of this RNA as consisting of a single hairpin and coined the term xe2x80x9chairpinxe2x80x9d Ribozyme (Bruening and colleagues use the term xe2x80x9cpaperclipxe2x80x9d for this ribozyme motif). Continuing experiments suggest an increasing number of similarities between the hairpin and hammerhead ribozymes in respect to both binding of target RNA and mechanism of cleavage.
Hepatitis Delta Virus (HDV) is a virus whose genome consists of single-stranded RNA. A small region (about 80 nucleotides) in both the genomic RNA, and in the complementary anti-genomic RNA, is sufficient to support self-cleavage. In 1991, Been and Perrotta proposed a secondary structure for the HDV RNAs that is conserved between the genomic and anti-genomic RNAs and is necessary for catalytic activity. Separation of the HDV RNA into xe2x80x9cribozymexe2x80x9d and xe2x80x9csubstratexe2x80x9d portions has recently been achieved by Been. Been has also succeeded in reducing the size of the HDV ribozyme to about 60 nucleotides.
Table I lists some of the characteristics of the ribozymes discussed above.
Eckstein et al., IntematiOnal Publication No. WO 92/07065; Perrault et al., Nature 1990, 344:565; Pieken et al., Science 1991, 253:314; Usman and Cedergren, Trends in Biochem. Sci. 1992, 17:334; and Rossi et al., International Publication No. WO 91/03162, describe various chemical modifications that can be made to the sugar moieties of enzymatic nucleic acid molecules.
Usman, et al., WO 93/15187 in discussing modified structures in ribozymes states:
It should be understood that the linkages between the building units of the polymeric chain may be linkages capable of bridging the units together for either in vitro or in vivo. For example the linkage may be a phosphorous containing linkage, e.g., phosphodiester or phosphothioate, or may be a nitrogen containing linkage, e.g., amide. It should further be understood that the chimeric polymer may contain non-nucleotide spacer molecules along with its other nucleotide or analogue units. Examples of spacer molecules which may be used are described in Nielsen et al. Science, 254:1497-1500 (1991).
Jennings et al., WO 94/13688 while discussing hammerhead ribozymes lacking the usual stem II base-paired region state:
One or more ribonucleotides and/or deoxydbonucleotides of the group (X)m, [stem II] may be replaced, for example, with a linker selected from optionally substituted polyphosphodiester (such as poly(1-phospho-3- propanol)), optionally substituted alkyl, optionally substituted polyamide, optionally substituted glycol, and the like. Optional substituents are well known in the art, and include alkoxy (such as methoxy, ethoxy and propoxy), straight or branch chain lower alkyl such as C1-C5 alkyl), amine, aminoalkyl (such as amino C1-C5 alkyl), halogen (such as F, Cl and Br) and the like. The nature of optional substituents is not of importance, as long as the resultant endonuclease is capable of substrate cleavage.
Additionally, suitable linkers may comprise polycyclic molecules, such as those containing phenyl or cyclohexyl rings. The linker (L) may be a polyether such as polyphosphopropanediol, polyethyleneglycol, a bifunctional polycyclic molecule such as a bifunctional pentalene, indene, naphthalene, azulene, heptalene, biphenylene, asymindacene, sym-indacene, acenaphthylene, fluorene, phenalene, phenanthrene, anthracene, fluoranthene, acephenathrylene, aceanthrylene, triphenylene, pyrene, chrysene, naphthacene, thianthrene, isobenzofuran, chromene, xanthene, phenoxathiin, indolizine, isoindole, 3-H-indole, indole, 1-H-indazole, 4-H-quinolizine, isoquinoline, quinoline, phthalazine, naphthyridine, quinoxaline, quinazoline, cinnoline, pteridine, 4-xcex1H-carbzole, carbazole, B-carboline, phenanthridine, acridine, perimidine, phenanthroline, phenazine, phenolthiazine, phenoxazine, which polycyctic compound may be substituted or modified, or a combination of the polyethers and the polycyclic molecules.
The polycyclic molecule may be substituted of polysubstituted with C1-C5 alkyl, alkenyl, hydroxyalkyi, halogen of haloalkyl group or with Oxe2x80x94A or CH2xe2x80x94Oxe2x80x94A wherein A is H or has the formula CONRxe2x80x2Rxe2x80x3 wherein Rxe2x80x2 and Rxe2x80x3 are the same or different and are hydrogen or a substituted or unsubstituted C1-C6 alkyl, aryl, cycloalkyl, or heterocyclic group; or A has the formula xe2x80x94Mxe2x80x94NRxe2x80x2 Rxe2x80x3 wherein Rxe2x80x2 and Rxe2x80x3 are the same or different and are hydrogen, or a C1-C5 alkyl, alkenyl, hydroxyalkyl, or haloalkyl group, wherein the halo atom is fluorine, chlorine, bromine, or iodine atom; and xe2x80x94Mxe2x80x94 is an organic moiety having 1 to 10 carbon atoms and is a branched or straight chain alkyl, aryl, or cycloalkyl group.
In one embodiment, the linker is tetraphosphopropanediol or Pentaphosphopropanediol. In the case of polycyclic molecules there will be preferably 18 or more atoms bridging the nucleic acids. More preferably their will be from 30 to 50 atoms bridging, see for Example 5. In another embodiment the linker is a bifunctional carbazole or bifunctional carbazole linked to one or more polyphosphoropropanediol.
Such compounds may also comprise suitable functional groups to allow coupling through reactive groups on nucleotides.xe2x80x9d
This invention concerns the use of non-nucleotide molecules as spacer elements at the base of double-stranded nucleic acid (e.g., RNA or DNA) stems (duplex stems) or more preferably, in the single-stranded regions, catalytic core, loops, or recognition arms of enzymatic nucleic acids. Duplex stems are ubiquitous structural elements in enzymatic RNA molecules. To facilitate the synthesis of such stems, which are usually connected via single-stranded nucteotide chains, a base or base-pair mimetic may be used to reduce the nucleotide requirement in the synthesis of such molecules, and to confer nuclease resistance (since they are non-nucleic acid components). This also applies to both the catalytic core and recognition arms of a ribozyme. In particular abasic nucleotides (i.e., moieties lacking a nucleotide base, but having the sugar and phosphate portions) can be used to provide stability within a core of a ribozyme, e.g., at U4 or N7 or a hammerhead structure shown in FIG. 1.
Thus, in a first aspect, the invention features an enzymatic nucleic acid molecule having one or more non-nucleotide moieties, and having enzymatic activity to cleave an RNA or DNA molecule.
Examples of such non-nucleotide mimetics are shown in FIG. 6 and their incorporation into hammerhead ribozymes is shown in FIG. 7. These non-nucleotide linkers may be either polyether, polyamine, polyamide, or polyhydrocarbon compounds. Specific examples include those described by Seela and Kaiser, Nucleic Acids Res. 1990, 18:6353 and Nucleic Acids Res. 1987, 15:3113; Cload and Schepartz, J. Am. Chem. Soc. 1991, 113:6324; Richardson and Schepartz, J. Am. Chem. Soc. 1991, 113:5109; Ma et al., Nucleic Acids Fles. 1993,21:2585 and Biochemistry 1993, 32:1751; Durand et al., Nucleic Acids Res. 1990, 18:6353; McCurdy et al., Nucleosides and Nucleotides 1991, 10:287; Jxc3xa4schke et at., Tetrahedron Lett. 1993, 34:301; Ono et al., Biochemistry 1991, 30:9914; Arnold et aL, International Publication No. WO 89/02439 entitled xe2x80x9cNon-nucleotide Linking Reagents for Nucleotide Probesxe2x80x9d; and Ferentz and Verdine, J. Am. Chem. Soc. 1991, 113:4000, all hereby incorporated by reference herein.
In preferred embodiments, the enzymatic nucleic acid includes one or more stretches of RNA, which provide the enzymatic activity of the molecule, linked to the non-nucleotide moiety.
By the term xe2x80x9cnon-nucleotide"" is meant any group or compound which can be incorporated into a nucleic acid chain in the place of one or more nucleotide units, including either sugar and/or phosphate substitutions, and allows the remaining bases to exhibit their enzymatic activity. The group or compound is abasic in that it does not contain a commonly recognized nucleotide base, such as adenine, guanine, cytosine, uracil or thymine. It may have substitutions for a 2xe2x80x2 or 3xe2x80x2 H or OH as described in the art. See Eckstein et al. and Usman et al., supra.
In preferred embodiments, the enzymatic nucleic acid includes one or more stretches of RNA, which provide the enzymatic activity of the molecule, linked to the non-nucleotide moiety. The necessary ribonucleotide components are known in the art, see, e.g., Usman, supra and Usman et al., Nucl. Acid. Symp. Series 31:163, 1994.
As the term is used in this application, non-nucleotide-containing enzymatic nucleic acid means a nucleic acid molecule that contains at least one non-nucleotide component which replaces a portion of a ribozyme, e.g., but not limited to, a double-stranded stem, a single-stranded xe2x80x9ccatalytic corexe2x80x9d sequence, a single-stranded loop or a single-stranded recognition sequence. These molecules are able to cleave (preferably, repeatedly cleave) separate RNA or DNA molecules in a nucleotide base sequence specific manner. Such molecules can also act to cleave intramoleculady if that is desired. Such enzymatic molecules can be targeted to virtually any RNA transcript. Such molecules also include nucleic acid molecules having a 3xe2x80x2 or 5xe2x80x2 non-nucleotide, useful as a capping group to prevent exonuclease digestion.
Enzymatic molecules of this invention act by first binding to a target RNA or DNA. Such binding occurs through the target binding portion of the enzyme which is held in close proximity to an enzymatic portion of molecule that acts to cleave the target RNA or DNA. Thus, the molecule first recognizes and then binds a target nucleic acid through complementary base-pairing, and once bound to the correct site, acts enzymatically to cut the target. Strategic cleavage of such a target will destroy its ability to direct synthesis of an encoded protein. After an enzyme of this invention has bound and cleaved its target it is released from that target to search for another target, and can repeatedly bind and cleave new targets.
The enzymatic nature of an enzyme of this invention is advantageous over other technologies, such as antisense technology (where a nucleic acid molecule simply binds to a nucleic acid target to block its translation) since the effective concentration of the enzyme necessary to effect a therapeutic treatment is lower than that of an antisense oligonucleotide. This advantage reflects the ability of the enzyme to act enzymatically. Thus, a single enzyme molecule is able to cleave many molecules of target RNA. In addition, the enzyme is a highly specific inhibitor, with the specificity of inhibition depending not only on the base pairing mechanism of binding, but also on the mechanism by which the molecule inhibits the expression of the RNA to which it binds. That is, the inhibition is caused by cleavage of the target and so specificity is defined as the ratio of the rate of cleavage of the targeted nucleic acid over the rate of cleavage of non-targeted nucleic acid. This cleavage mechanism is dependent upon factors additional to those involved in base pairing. Thus, it is thought that the specificity of action of an enzyme of this invention is greater than that of antisense oligonucleotide binding the same target site.
By xe2x80x9ccomplementarityxe2x80x9d is meant a nucleic acid that can form hydrogen bond(s) with other RNA sequence by either traditional Watson-Crick or other non-traditional types (for example, Hoogsteen type) of base-paired interactions.
By the phrase enzyme is meant a catalytic non-nucleotide-containing nucleic acid molecule that has complementarity in a substrate-binding region to a specified nucleic acid target, and also has an enzymatic activity that specifically cleaves RNA or DNA in that target. xe2x80x9cThat is, the enzyme is able to intramolecularly or intermolecularly cleave RNA or DNA and thereby inactivate a target RNA or DNA molecule. This complementarity functions to allow sufficient hybridization of the enzymatic molecule to the target RNA or DNA to allow the cleavage to occur. One hundred percent complementarity is preferred, but complementarity as low as 50-75% may also be useful in this invention.
In preferred embodiments of this invention, the enzymatic nucleic acid molecule is formed in a hammerhead or hairpin motif, but may also be formed in the motif of a hepatitis delta virus, group I intron or RNaseP RNA (in association with an RNA guide sequence) or Neurospora VS RNA. Examples of such hammerhead motifs are described by Rossi et al., 1992, Aids Research and Human Retroviruses 8, 183, of hairpin motifs by Hampei et al., EP0360257, Hampel and Tritz, 1989 Biochemistry 28, 4929, and Hampel et al., 1990 Nucleic Acids Res. 18, 299, and an example of the hepatitis delta virus motif is described by Perrotta and Been, 1992 Biochemistry 31, 16; of the RNaseP motif by Guerrier-Takada et al., 1983 Cell 35, 849, Neurospora VS RNA ribozyme motif is described by Collins (Saville and Collins, 1990 Cell 61, 685-696; Saville and Collins, 1991 Proc. Natl. Acad. Sci. USA 88, 8826-8830; Collins and Olive, 1993 Biochemistry 32, 2795-2799) and of the Group I intron by Cech et al., U.S. Pat. No. 4,987,071. These specific motifs are not limiting in the invention and those skilled in the art will recognize that all that is important in an enzymatic nucleic acid molecule of this invention is that it has a specific substrate binding site which is complementary to one or more of the target gene RNA regions, and that it have nucleotide sequences within or surrounding that substrate binding site which impart an RNA cleaving activity to the molecule.
The invention provides a method for producing a class of enzymatic cleaving agents which exhibit a high degree of specificity for the nucleic acid of a desired target. The enzyme molecule is preferably targeted to a highly conserved sequence region of a target such that specific treatment of a disease or condition can be provided with a single enzyme. Such enzyme molecules can be delivered exogenously to specific cells as required. In the preferred hammerhead motif the small size (less than 60 nucleotides, preferably between 30-40 nucleotides in length) of the molecule allows the cost of treatment to be reduced compared to other ribozyme motifs.
Synthesis of nucleic acids greater than 100 nucleotides in length is difficult using automated methods, and the therapeutic cost of such molecules is prohibitive. In this invention, small enzyme motifs (e.g., of the hammerhead structure) are used for exogenous delivery. The simple structure of these molecules increases the ability of the enzyme to invade targeted regions of mRNA structure. Unlike the situation when the hammerhead structure is included within longer transcripts, there are no non-enzyme flanking sequences to interfere with correct folding of the enzyme structure or with complementary regions.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.